This invention relates generally to matrix amplifying circuits and more particularly to a matrix amplifying circuit which is especially adapted for matrixing a sum signal and a difference signal taken from a multichannel record disc reproducing system and for thereby obtaining separate channel signals.
A discrete 4-channel record disc has been previously described in a granted U.S. Pat. No. 3,686,741. Two sum signals and two difference signals are obtained by a matrix circuit used in the recording system. A direct wave sum signal and an angle-modulated wave (obtained by angle modulating a carrier wave of 30 KHz with a difference signal) are recorded in a superimposed state on each wall of a disc sound groove. In the reproducing system, a pickup cartridge reproduces the direct wave sum signal and the angle-modulated wave difference signal which are thereafter separated from the superimposed signals. The angle-modulated wave difference signal is demodulated. The above mentioned sum signal and difference are matrix by a matrix circuit and then led out separately as the signals of the different channels.
In one known matrix circuit, used in a multichannel record disc reproducing system as described above, the difference signal of the first and second channels is fed to the base of a transistor. The sum signal of the first and second channels is added to a difference signal of opposite phase, by way of a plurality of suitable resistors, respectively. The opposite phase signals are derived from the collector of the transistor by way of a first capacitor. A difference signal of the same phase is derived from the emitter of the transistor, by way of a second capacitor. First and second channel signals are respectively obtained separately. The same circuit is used also for obtaining the third and fourth channels.
In this known matrix circuit, in actual practice, however, there are differences in the output impedances of the emitter and collector of the transistor. Furthermore, there are errors in the resistance values between the various resistors. For this reason, there are differences in the gains for the difference and the sum signals, as measured between the input side and the output side, for the signals of the different channels. Consequently, accurate addition and subtraction operations cannot be carried out, and deviations occur between individual output channel signals.
Furthermore, as a result of the first and second capacitors for interstage coupling, a phase shift occurs in the low-frequency band of the signal. It is difficult to carry out accurate addition and subtraction operations.